The long term objective of the research program is to gain a better understanding of the molecular-genetic controls of myelopoiesis and leukemogenesis. An experimental approach to dissect the regulation of differentiation and how differentiation blocks can arise is to genetically manipulate myeloid cells, and to analyze these cells at the cellular, molecular and genetic level. It has been shown that, in addition to constitutive c-myc causing Interleukin-6 (IL-6) treated M1myc cells to arrest at an intermediate stage of differentation, a portion of the IL-6 treated proliferating population of cells undergoes apoptotic cell death. Similar observations were made for the nonleukemic 32D cell line and normal myeloid cells from bone marrow (BM). It is hypothesized that the effects of c-myc on hematopoietic development are mediated via c-myc target genes and genes induced by differentiation factors. The c-myc target gene cdc25A appears to mediate some effects of c-myc on differentiation, and the genes Gadd45, MyD116 and CR6, induced by myeloid differentiation factors, participate in the apoptotic response associated with the block in differentiation. This proposal intends to extend these novel findings, to decipher how these gene products participate in the myc-mediated effects on myeloid development, and to identify other players. Since c-myc is a member of a transcription factor network which regulates c-myc target genes, to understand how c-myc carries out its functions it is necessary to understand this network of interacting factors. The proposed studies will employ the M1 and 32D cell lines, as well as myeloid cells from BM. The specific aims of this proposal are: Aim I: Molecular-Genetic Dissection of c-myc-mediated block in differentiation and growth arrest: A role for the c-myc target gene cdc25A and other target genes. Aim II: Molecular-genetic Dissection of the Apoptotic Response Associated with the Myc- Mediated Block in Terminal Differentiation: A Role for MyD and Gadd Genes. Aim III: To ascertain the role of the network of c- myc interacting factors in regulating differentiation, growth arrest and apoptosis. These studies should reveal how perturbing c-myc, and, therefore, normal cell cycle controls, can block differentiation, alter cell survival, and promote leukemogenesis. These investigations should contribute to a greater understanding of the genetic events involved in the pathogenesis of different leukemias, ultimately aiding in diagnosis, prognosis and eventual therapy.